In the various manufacturing processes for electric devices, plasma generated by a microwave is used. In order to generate stabilized plasma, impedance-matching is necessary between a microwave source and plasma, which is a load. Since plasma is unstable as a load, normally it is necessary to control impedance.
In FIG. 1, plasma generated by a microwave is illustrated. Within this plasma generated by the microwave, there is a standing wave generated by an incident wave and corresponding reflected wave when impedance miss-matches. In plasma impedance-match measurement, conventionally, a standing wave in the above-mentioned plasma is directly measured using the Four-Point Prove Method where a probe is inserted in plasma. There are, however, the following problems:
(1) Plasma is unstable as a microwave circuit load;
(2) Since the reflected amount of the microwave from plasma is small, the corresponding detected level at around impedance-matched region becomes small, making it impossible to distinguish from noises (S/N ratio worsens); and
(3) Since the error derived from the poor processing precision and/or the assembly precision of each probe influences detection precision, etc., control precision for impedance matching worsens, making it difficult to provide stabilized plasma.